The use of microwave energy to cook a variety of prepared foodstuffs for consumption is increasing. One of the problems associated with such cooking is to achieve an even distribution of heat in the cooked product, particularly in frozen products. For example, foodstuffs like macaroni and cheese and lasagna, which are cooked in trays, tend to be hotter in the peripheral regions of the tray than in the central regions when cooked by exposure to microwave energy.
The trays in which the foodstuffs are cooked generally are formed of rigid polymeric materials or polymeric material-coated paperboard.
It has previously been proposed in U.S. Pat. No. 4,351,997 to provide a modified form of tray structure to attempt to provide a more even heating of foodstuff in the tray when exposed to microwave radiation. This prior art tray has a bottom wall of microwave-transparent material and an upwardly-extending peripheral wall which is outwardly curved at its upper end to define a horizontally-extending peripheral rim.
The rim is partly or completely coated with a material which is reflective and opaque to microwave radiation, such as aluminum foil. The peripheral wall also is partly or completely coated with the foil material. The effect of this aluminum foil coating on the wall is to provide reflection of microwave energy towards the centre of the tray.
In this structure, therefore, both the outer surface of the rim and the walls are coated, at least partially with aluminum foil. The arrangement as described in this prior art provides aluminum metal directly in contact with the food in the tray, which is unsatisfactory in many foodstuff applications, because of the possibility of contamination of the foodstuff. The coating on the rim is said to protect the paperboard rim from being deteriorated in use of the tray.
U.S. Pat. No. 4,626,641 describes an embodiment in which a similar structure is provided. In addition to the provision of aluminum foil in the side wall of a tray, the foil also extends into the base of the container but leaves a rectangular open area in the bottom wall.
While both of these prior art structures provide an improved uniformity of heating of the foodstuff in a microwave oven, there remains a significant spread of temperature between the edge regions and core regions of the foodstuff.